U.S. patent number 7,787,149 [Application Number 11/487,488] was granted by the patent office on 2010-08-31 for method of generating color conversion table, information generation apparatus, recording medium, and image processing apparatus.
This patent grant is currently assigned to Konica Minolta Business Technologies, Inc.. Invention is credited to Shuji Ichitani.
United States Patent |
7,787,149 |
Ichitani |
August 31, 2010 |
Method of generating color conversion table, information generation
apparatus, recording medium, and image processing apparatus
Abstract
A color conversion table for converting M color signals of an
input system into N color signals of an output system is disclosed.
The color conversion table is prepared by and obtained based on a
calculation target point corrected by moving a position of the
maximum gradation of each fundamental color of the output system to
a position of the maximum gradation of the fundamental color of the
input system and on calculation target points corrected for
gradations other then the maximum gradation of each fundamental
color by moving positions of the gradations of the fundamental
color in output system to the positions of the gradations of the
fundamental color in the input system.
Inventors: |
Ichitani; Shuji (Hachioji,
JP) |
Assignee: |
Konica Minolta Business
Technologies, Inc. (Tokyo, JP)
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Family
ID: |
38533036 |
Appl.
No.: |
11/487,488 |
Filed: |
July 17, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070223016 A1 |
Sep 27, 2007 |
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Foreign Application Priority Data
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Mar 23, 2006 [JP] |
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2006-081469 |
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Current U.S.
Class: |
358/1.9;
358/521 |
Current CPC
Class: |
G06T
11/001 (20130101); H04N 1/6025 (20130101); H04N
1/6058 (20130101) |
Current International
Class: |
G03F
3/08 (20060101) |
Field of
Search: |
;358/1.1,1.9,3.23,500,501,518,521,523 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lee; Thomas D
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, LLP
Claims
What is claimed is:
1. A method of using an image processing apparatus for generating a
color conversion table for converting M color signals of an input
system into N color signals of an output system, comprising the
processor steps of: correcting, for each fundamental color, a
calculation target point by moving a position, on chromatic
coordinates, of the maximum gradation of the fundamental color of
the output system to a position, on the chromatic coordinates, of
the maximum gradation of fundamental color of the input system; and
correcting, for each gradation other than the maximum gradation of
each fundamental color, a calculation target point of the
fundamental color of the output system by moving a position, on the
chromatic coordinates, of the gradation of the fundamental color of
the output system to a position, on the chromatic coordinates, of
the gradation of the fundamental color of the input system wherein
correcting, for each fundamental color, is performed by calculating
a first moving distance by obtaining a difference between the
position of the maximum gradation of the fundamental color of the
output system and the position of the maximum gradation of the
fundamental color of the input system; and wherein correcting, for
each gradation other than the maximum gradation of each fundamental
color, is performed by calculating a second moving distance by
obtaining a difference between a position of the gradation of the
fundamental color of the output system and a position of the
gradation of the fundamental color of the input system.
2. The method of using an image processing apparatus for generating
a color conversion table of claim 1, wherein the color signals of
the input system are image signals of red, green and blue, and the
color signals of the output system are image signals of cyan,
magenta, yellow and black.
3. An information generating apparatus for generating a color
conversion table to be used for converting M color signals of an
input system into N color signals of an output system, comprising:
a correction section which corrects, for each fundamental color, a
calculation target point by moving a position, on chromatic
coordinates, of the maximum gradation of the fundamental color of
the output system to a position, on the chromatic coordinates, of
the maximum gradation of the fundamental color of the input system
and corrects, for each gradation other than the maximum gradation
of each fundamental color, a calculation target point of the
fundamental color of the output system by moving a position, on the
chromatic coordinates, of the gradation of the fundamental color of
the output system to a position, on the chromatic coordinates, of
the gradation of the fundamental color of the input system, wherein
the correction section is configured to: calculate, for each
fundamental color, a first moving distance by obtaining a
difference between the position of the maximum gradation of the
fundamental color of the output system and the position of the
maximum gradation of the fundamental color of the input system; and
calculate, for each gradation other than the maximum gradation of
each fundamental color, a second moving distance by obtaining a
difference between a position of the gradation of the fundamental
color of the output system and a position of the gradations of the
fundamental color of the input system.
4. The information generating apparatus of claim 3, wherein the
color signals of the input system are image signals of red, green
and blue, and the color signals of the output system are image
signals of cyan, magenta, yellow and black.
5. A computer-readable storage medium storing a color conversion
table and a plurality of instructions that, when executed by an
image processing apparatus, are configured to convert M color
signals of an input system into N color signals of an output
system; wherein the color conversion table is obtained based on a
calculation target point corrected by moving a position, on
chromatic coordinates, of the maximum gradation of each fundamental
color of the output system to a position, on the chromatic
coordinates, of the maximum gradation of the fundamental color of
the input system, and based on calculation target points corrected
for gradations other than the maximum gradation of each fundamental
color by moving positions, on the chromatic coordinates, of the
gradations of the fundamental color in output system to the
positions, on the chromatic coordinates, of the gradations of the
fundamental color in the input system; wherein the calculation
target point of each fundamental color is corrected by calculating
a first moving distance by obtaining a difference between the
position of the maximum gradation of the fundamental color of the
output system and the position of the maximum gradation of the
fundamental color of the input system; and wherein the calculation
target point of each gradation other than the maximum gradation of
each fundamental color is corrected by calculating a second moving
distance by obtaining a difference between a position of the
gradation of the fundamental color of the output system and a
position of the gradation of the fundamental color of the input
system.
6. An image processing apparatus, comprising: the computer-readable
storage medium of claim 5 for storing the color conversion table;
and a processing section configured to convert M color signals of
an input system into N color signals of an output system based on
information stored in the color conversion table.
Description
This application is based on Japanese Patent Application No.
2006-081469 filed on Mar. 23, 2006, in Japanese Patent Office, the
entire content of which is hereby incorporated by reference.
TECHNICAL FIELD
The present invention relates to methods of generating a color
conversion table for converting the colors of M input color signals
into N output color signals, information generation apparatuses for
generating such a table, recording medium, and image processing
apparatuses provided with the recording medium.
BACKGROUND
In recent years, the use of scanners, monitors (color display
devices), color printers, color copying machines, and all-in-one
units having these is increasing. This type of color image forming
apparatuses are often equipped with a three-dimensional color
information conversion table (a three-dimensional look Up table,
also referred to hereinafter as RGB.fwdarw.CMYK3D-LUT) that
converts the image information of signal processing system of the
colors red (R), green (G), and blue (B) into an image data of the
CMYK signal processing system. This is because, in the image
forming apparatus, the configuration is such that the operations
are made based on image data of the CMYK signal processing
system.
The RGB.fwdarw.CMYK3D-LUT is prepared, for example, from the
measured color values (XYZ or Lab) of the original document with
n.sup.3 patches in which n patches are arranged so that the
intensity of each of the three colors RGB increases, and from the
scanner signal (RGB) by carrying out matrix processing and
interpolation computation processing, and the RGB.fwdarw.CMYK3D-LUT
is for converting the RGB signals to XYZ output signals or Lab
output signals.
Relating to the image forming apparatus handling the above
described color image data, an image processing apparatus has been
disclosed in Japanese Unexamined Laid-Open Patent Publication No.
H07-236069. According to this image processing apparatus, the color
image data inputted from the input device is subjected to color
gamut compression processing to suit the color reproduction region
of the output device, and the output image data after this color
gamut compression is outputted to the output device.
FIG. 32 is a flow chart showing an example of preparing the
RGB.fwdarw.CMYK3D-LUT of a conventional example. To begin with, in
Step R1 in the flow chart shown in FIG. 32, the input data and the
Lab data are read out from the prescribed image memory and expanded
in the RAM (Random Access Memory), etc. The input data and the Lab
data are expanded in the RAM and the RGB.fwdarw.Lab3D-LUT is
prepared.
Next, in Step R2, the CMY data is read in from the image memory and
expanded in the RAM. The CMY data is expanded in the RAM and the
CMYK.fwdarw.Lab4D-LUT is prepared. Next, the operation moves on to
Step R3 in which the CMY.fwdarw.Lab3D-LUT is prepared from the
CMYK.fwdarw.Lab4D-LUT based on a GCR (Gray-Component Replacement)
algorithm.
Thereafter, the operation moves on to Step R4 in which the Lab
value to calculate in the Lab coordinate system is set. At this
time, the CMY value corresponding to each of the Lab values of
RGB.fwdarw.Lab3D-LUT is computed from the CMY.fwdarw.Lab3D-LUT.
Next, the operation moves on to Step R5 in which the Lab values set
(inputted) earlier are searched from the CMY.fwdarw.Lab3D-LUT, and
a judgment is made as to whether or not the Lab value is within the
color gamut of the CMY.fwdarw.Lab3D-LUT (color gamut inside or
outside judgment process). When the calculation target point Pin is
judged, by this color gamut inside or outside judgment process, to
be outside the color gamut of the printer, the operation moves on
to Step R6 in which the compression process is executed.
Further, in Step R5 described above, if the calculation target
point Pin is judged to be within the color gamut of the printer,
the operation moves on to Step R7 in which the CMY values are
searched in the CMY.fwdarw.Lab3D-LUT using the Lab value set
earlier by inclusion judgment, and the CMY values are calculated
corresponding to this inclusion judgment. Thereafter, the operation
moves on to Step R8 in which a judgment is made as to whether or
not all the computation processing of the CMY values corresponding
to the Lab value set earlier has been completed.
If all the computation has not been completed, the operation
returns to Step R4 and the processing described above is repeated.
If the computation processing of the CMY values for all the Lab
values that have been inputted earlier is completed, the
RGB.fwdarw.CMY3D-LUT is prepared. Next, the operation moves on to
Step R9 in which the RGB.fwdarw.CMY3D-LUT is converted to the
RGB.fwdarw.CMYK3D-LUT by the GCR. In this manner it is possible to
prepare the RGB.fwdarw.CMYK3D-LUT (this is referred to hereafter as
the conventional example 1).
However, the following problems are present in an image processing
apparatus according to a conventional example.
i. In an image processing apparatus as in the Japanese Laid-Open
Patent Publication or in the conventional example 1, regarding the
reproduction of the six fundamental colors of RGBCMY, as a method
of eliminating color mixing, there is the method of moving the
fundamental color point of the input side to the output side, and
carrying out linear interpolation for other parts (hereinafter
referred to as the conventional example 2). In FIG. 33(A), the
filled circles denote the fundamental color gradations on the input
side. The open circles are the gradations on the fundamental color
output side. In actuality, the fundamental color gradation is not
as indicated by a straight line in FIG. 33(A) but is non-linear as
indicated in FIG. 33(B).
Because of this movement and linear interpolation, regarding the
reproduction of fundamental colors, although in appearance it is
thought that color mixing is eliminated, the actual color
gradations of printers and scanners are usually not linear in the
Lab space, but are bent as shown in FIG. 33(B). As a consequence,
the method of moving the fundamental colors and carrying out linear
interpolation for other parts results in color mixing with the
intermediate gradation parts of fundamental colors not matching as
is shown in FIG. 34(A) and FIG. 34(B). In the conventional example
2, the filled circles denote the fundamental color gradations on
the input side. The open circles are the gradations on the
fundamental color output side. In actuality, the fundamental color
gradation is not as indicated by a straight line in FIG. 34(A) but
is non-linear as indicated in FIG. 34(B). In other words, even the
maximum gradation of the output side fundamental color is made to
match with the maximum gradation of the input side fundamental
color, there will be a difference in the color gamuts of the input
side fundamental colors and the color gamuts of the output side
fundamental colors. In FIG. 34(B), the horizontal axis corresponds
to the color saturation a* of magenta (M) color and the vertical
axis corresponds to the color saturation b* of yellow (Y) color.
Therefore, although the fundamental color points match, other
gradations do not necessarily match. This becomes the cause of
color mixing.
ii. When color mixing of other colors occurs in the reproduction of
fundamental colors, not only the image is bad to see, but also can
cause wasteful consumption of the toner. For example, when
reproducing the fundamental color R, instead of being reproduced
using only colors Y and M, if the color C is also added, the
consumption of the toner of that color C will be wasteful.
SUMMARY
In view of forgoing, one embodiment according to one aspect of the
present invention is a method for generating a color conversion
table for converting M color signals of an input system into N
color signals of an output system, comprising the steps of:
correcting, for each fundamental color, a calculation target point
by moving a position of the maximum gradation of the fundamental
color of the output system to a position of the maximum gradation
of the fundamental color of the input system; and
correcting, for each gradation other than the maximum gradation of
each fundamental color, a calculation target point of the
fundamental color of the output system by moving a position of the
gradation of the fundamental color of the output system to a
position of the gradation of the fundamental color of the input
system.
According to another aspect of the present invention, another
embodiment is an information generation apparatus for converting M
color signals of an input system into N color signals of an output
system, comprising:
a correction section which corrects, for each fundamental color, a
calculation target point by moving a position of the maximum
gradation of the fundamental color of the output system to a
position of the maximum gradation of the fundamental color of the
input system and corrects, for each gradation other than the
maximum gradation of each fundamental color, a calculation target
point of the fundamental color of the output system by moving a
position of the gradation of the fundamental color of the output
system to a position of the gradation of the fundamental color of
the input system.
According to another aspect of the present invention, another
embodiment is a computer-readable recording medium storing a color
conversion table for converting M color signals of an input system
into N color signals of an output system, wherein the color
conversion table is obtained based on a calculation target point
corrected by moving a position of the maximum gradation of each
fundamental color of the output system to a position of the maximum
gradation of the fundamental color of the input system and on
calculation target points corrected for gradations other than the
maximum gradation of each fundamental color by moving positions of
the gradations of the fundamental color in output system to
positions of the gradations of the fundamental color in the input
system.
According to another aspect of the present invention, another
embodiment is an image processing apparatus, comprising the
aforementioned computer-readable recording medium for color
converting M color signals of an input system into N color signals
of an output system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an example of configuration of an
image processing apparatus 100 as a preferred embodiment.
FIG. 2 is a color coordinates diagram showing an example of
development of the CMY.fwdarw.Lab3D-LUT and the
RGB.fwdarw.Lab3D-LUT in the Lab coordinates system constituting
RGB.fwdarw.CMYK3D-LUT.
FIG. 3 is a block diagram showing a sample configuration of the
information generation apparatus 70.
FIG. 4 is a flow chart (main routine) showing an example of
generating the RGB.fwdarw.CMYK3D-LUT with the calculation target
points compensated for each of the six fundamental colors and that
is executed in the information generation apparatus 70.
FIG. 5 is a flow chart showing an example of generating the
CMY.fwdarw.CMYK3D-LUT using a GCR algorithm.
FIG. 6 is a flow chart (subroutine) showing an example of
generating the CMY.fwdarw.Lab3D-LUT.
FIG. 7 is a diagram showing an example of the relationship between
the input side fundamental color point of the CMYRGB colors in the
Lab coordinate system and the printer side fundamental color points
(Max).
FIG. 8 is a diagram showing an example of adjustment of the
fundamental color points in the Lab coordinate system.
FIG. 9 is a flow chart showing an example of adjusting the
positions of the six fundamental color points in the Lab coordinate
system.
FIGS. 10(A), 10(B) are diagrams showing an example of the
relationship between the error DiffL* in the lightness of the input
values and the output values of the six fundamental colors, their
chroma error DiffC*, and the color phase Hue.
FIG. 11 is a diagram showing an example of the relationship between
the error DiffH* in the hue of the input values and output values
of the six fundamental colors and that color phase Hue.
FIG. 12 is a conceptual diagram showing an example of adjusting the
fundamental color gradation position in the Lab coordinate
system.
FIG. 13 is a flow chart (subroutine) showing an example of
processing at the time of adjusting the fundamental color gradation
position.
FIGS. 14(A), 14(B) are diagrams showing an example of plotting the
input color lightness L* and the input chroma C* (0 to 100%) versus
chroma (0 to 100%).
FIG. 15 is a diagram showing an example of plotting the input hue
H* (0 to 100%) versus chroma (0 to 100%).
FIG. 16 is a diagram showing an example of judging the inclusion in
triangles in the hue versus chroma plane.
FIGS. 17(A), 17(B) are diagrams showing an example of inclusion
judgment in triangles and an example of triangular
interpolation.
FIG. 18 is a diagram showing an example of plotting the color
gradations after adjusting the six fundamental colors.
FIG. 19 is a diagram showing an example of color gamut surface
search of the calculation target point pin in the Lab coordinate
system.
FIG. 20 is a diagram showing an example of color gamut surface
search in the CMY coordinate system.
FIG. 21 is a diagram showing an example of inclusion judgment in
the Lab coordinate system.
FIG. 22 is a diagram showing an example of inclusion judgment in
the CMY coordinate system.
FIG. 23 is a flow chart showing an example of evaluation of the
fundamental color reproduction capability.
FIG. 24 is a diagram showing a sample configuration of a color
patch of CMYRGB colors.
FIGS. 25(A), 25(B) are tabular diagrams showing the relationship
(1) between the sample number of the color patch and the color
saturation gradation.
FIGS. 26(A), 26(B) are tabular diagrams showing the relationship
(2) between the sample number of the color patch and the color
saturation gradation.
FIGS. 27(A), 27(B) are tabular diagrams showing the relationship
(3) between the sample number of the color patch and the color
saturation gradation.
FIG. 28 is a diagram showing an example of the relationship between
the input values (white-red maximum) and the output values [%] of
the colors C, M, Y, and K.
FIGS. 29(A)-29(F) are diagrams showing an example of input-output
conversion of the six fundamental colors in the conventional
example 1.
FIGS. 30(A)-30(F) are diagrams showing an example of input-output
conversion of the six fundamental colors in the conventional
example 2.
FIGS. 31(A)-31(B) are diagrams showing an example of input-output
conversion of the six fundamental colors in the present preferred
embodiment.
FIG. 32 is a flow chart showing an example of generating the
RGB.fwdarw.CMYK3D-LUT in the conventional examples.
FIGS. 33(A), 33(B) are diagrams showing an example of the
relationship between the input side fundamental color gradation and
the output side fundamental color gradation in the conventional
example 1.
FIGS. 34(A), 34(B) are diagrams showing an example of the
relationship between the input side fundamental color gradation and
the output side fundamental color gradation in the conventional
example 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The method of generating the color conversion table, the
information generation apparatus, the recording medium, and the
image processing apparatus according to a preferred embodiment of
the present invention are described in the following while
referring to the drawings.
FIG. 1 is a block diagram showing an example of configuration of an
image processing apparatus 100 as a preferred embodiment. The image
processing apparatus 100 shown in FIG. 1 is an apparatus that
carries out the conversion processing of the color image signals of
the input system 20 with a prescribed number of bits into an N-bit
color image signal of the output system 30. In the present example,
for example, a scanner is placed in a three dimensional signal
processing system of red (R), green (G), and blue (B) which is an
example of an input system (hereinafter referred to as the RGB
signal processing system 20), and a printer is placed in the four
dimensional signal processing system of cyan (C), magenta (M),
yellow (Y), and black (K) (hereinafter referred to as the CMYK
signal processing system 30) which is an example of an output
system.
The image processing apparatus 100 is placed between the scanner
and the printer, and, for example, operates so as to convert the
color image signals R, G, B of the RGB signal processing system 20
into the color signals C, M, Y, K of the CMYK signal processing
system 30. In this example, for the sake of convenience, although
the image processing apparatus 100 has been placed between the
scanner and the printer, of course, it is also possible to install
the image processing apparatus 100 within the scanner, or to
install it within the printer. Further, it is also possible that
the scanner, image processing apparatus, and the printer are
configured integrally as an all-in-one unit.
The image processing apparatus 100 is configured to include a ROM
40 and a color conversion interpolation unit 50. The ROM 40
constitutes an example of a recording medium, and stores the
RGB.fwdarw.CMYK3D-LUT. The ROM 40 is one in which is recorded the
color conversion table for converting the color image signals R, G,
and B of the three color RGB input system into the color image
signals C, M, Y, and K of the four color CMYK output system, and
the ROM 40 stores the color conversion table computed at the
calculation target points at which the position of the maximum
gradation of the output system six fundamental colors (each of the
colors R, Y, G, C, B, and M) is moved to the position of the
maximum gradation of input system six fundamental colors (each of
the colors R, Y, G, C, B, and M) and correction is carried out, and
computed at the calculation target points at which the positions
other than that of the maximum gradation of output system
fundamental colors are moved to the positions other than that of
the maximum gradation of input system fundamental colors and
correction is carried out at each gradation of that output system
fundamental colors.
The color conversion interpolation unit 50 is configured so that
the N-bit color image signals C, M, Y, K of the CMYK processing
system is outputted by interpolating between the color image
signals R, G, B with a prescribed number of bits of the RGB signal
processing system based on the N-bit CMYK values read out from the
ROM 40. The color image signals of the input side signal processing
system can also be, apart from RGB signals, CMYK, CMY, Lab, or XYZ
signals, etc., and the color image signals of the output side
signal processing system can be, apart from CMYK signals, RGB, CMY,
Lab, or XYZ signals, etc.
The color conversion interpolation unit 50, for example, is
configured so that all the color image signals R, G, B of the RGB
signal processing system which is the input system that is outside
the printer color gamut and cannot be handled by the CMYK signal
processing system 30 which is the output system are color converted
and interpolated to the upper limit value or the lower limit value
of the printer color gamut handled by that CMYK signal processing
system, and the color image signals C, M, Y, K are outputted.
The RGB.fwdarw.CMYK3D-LUT is prepared by executing a step of
detecting whether or not the input RGB value of the calculation
target point is present outside the printer color gamut of the
color image signal handled by the CMYK signal processing system, a
step of executing the internal insertion processing mode if the
input RGB value of the calculation target point is present inside
the printer color gamut of the color image signal handled by the
CMYK signal processing system, or a step of executing the external
insertion processing mode if the input RGB value of the calculation
target point is present outside the printer color gamut of the
color image signal handled by the CMYK signal processing
system.
Here, the internal insertion processing mode is the processing of
obtaining the output CMYK value of the color image signal handled
by the CMYK signal processing system corresponding to the input RGB
values of the four peak points encircling the input RGB value of
the calculation target point when the input RGB value is expressed
by expanding the color image signals R, G, and B of the RGB signal
processing system.
In addition, the external insertion processing mode is the
processing of obtaining the output CMYK value of the color image
signal handled by the CMYK signal processing system corresponding
to the input RGB values of three peak points encircling the
calculation target point and the RGB value of the calculation
reference point, after extracting the calculation reference point
from the color image signals of the RGB signal processing system
expressed in the isochromatic three dimensional coordinate system,
and also, fixing that calculation reference point, and connecting
that calculation reference point with the calculation target point
by a straight line. This external insertion processing mode
includes the processing of obtaining the CMYK values less than 0
gradations and more than 2.sup.N gradations outside the printer
color gamut that cannot be handled by the CMYK signal processing
system.
According to the RGB.fwdarw.CMYK3D-LUT described above, in the case
of carrying out color conversion of the color image signals of the
RGB signal processing system into the color image signals C, M, Y,
and K of the CMYK signal processing system 30, the color image
signals R, G and B of the RGB signal processing system 20 outside
the printer color gamut and that cannot be handled by the CMYK
signal processing system 30 can be color converted and interpolated
with good reproducibility as CMYK values of the upper limit or the
lower limit of the printer color gamut handled by that CMYK signal
processing system 30.
In this manner, according to the image processing apparatus 100 as
a preferred embodiment, a ROM 40 is provided according to the
present preferred embodiment in which the calculation target points
are corrected for each of the six fundamental colors, the color
image signals R, G, and B of the three color RGB input system is
color converted into the color image signals C, M, Y, and K of the
four color CMYK output system.
Therefore, it is possible to carry out color conversion of the
color image signals R, G, and B of the three color RGB input system
into the color image signals C, M, Y, and K of the four color CMYK
output system without color mixing. Because of this, it is possible
to provide an image processing apparatus 100 that can reproduce
high quality color images.
Next, further detailed explanation is given about the
RGB.fwdarw.CMYK3D-LUT in which the calculation target point is
corrected for each of the six fundamental colors. FIG. 2 is a color
coordinates diagram showing an example of development of the
CMY.fwdarw.Lab3D-LUT and the RGB.fwdarw.Lab3D-LUT in the Lab
coordinates system constituting RGB.fwdarw.CMYK3D-LUT.
The vertical axis shown in FIG. 2 is the lightness L* axis, and the
horizontal axis is the chromaticity b* axis. The point at the
intersection of the lightness L* axis and the chromaticity b* axis
is the chromaticity a* axis. In FIG. 2, the first lattice shape I
is the RGB.fwdarw.Lab3D-LUT which is the 8-bit color image signals
R, G, and B of the RGB signal processing system expressed in 256
gradations. The outline of the lattice shape I indicates the
boundary of the color gamut that is handled by the RGB signal
processing system.
In this example, the lattice shape II developed so as to be
included inside the lattice shape I indicates the
CMY.fwdarw.Lab3D-LUT which is the 8-bit color image signals C, M,
Y, and K handled by the CMYK signal processing system and expressed
in 256 gradations. The outline of the lattice shape II indicates
the boundary of the color gamut (hereinafter referred to as the
printer color gamut) that is handled by the CMYK signal processing
system. The top corner part corresponds to a CMY value of [0, 0,
0], the left corner part to a CMY value of [255, 255, 0], the
bottom corner part to a CMY value of [255, 255, 255], and the right
corner part to a CMY value of [0, 0, 255].
In the RGB.fwdarw.Lab3D-LUT of the lattice shape I, the downward
right slanting inclined line (side) denotes the Y gradations (0 to
255) of the scanner and the downward left slanting inclined line
(side) denotes the B gradations (0 to 255) of the scanner. In the
CMY.fwdarw.Lab3D-LUT of the lattice shape II, the downward right
slanting inclined line (side) of the denotes the Y gradations (0 to
255) of the printer and the downward left slanting inclined line
(side) denotes the B gradations (0 to 255) of the printer. In both
the RGB.fwdarw.Lab3D-LUT and the CMY.fwdarw.Lab3D-LUT, the Y color
increases downwards to the right and the B color increases
downwards to the left.
In this example, the inside or outside judgment of whether to
execute the internal insertion processing mode or to execute the
external insertion processing mode described above is carried out
by setting that reference in the printer color gamut (the color
gamut handled by the CMYK signal processing system). Here, the
interior of the printer color gamut is defined as inside the
printer color gamut and its exterior is defined as outside the
printer color gamut.
Next, the method of generating the color conversion table in the
present preferred embodiment is described here. In this example,
the presumption is the case when the 8-bit color image signals R,
G, and B of the RGB signal processing system are converted into the
8-bit color image signals C, M, Y, and K of the CMYK signal
processing system. FIG. 3 is a block diagram showing a sample
configuration of the information generation apparatus 70. In the
present preferred embodiment, although the information generation
apparatus 70 has been configured as a separate entity from the
information processing apparatus 100, it can also be configured to
be an integral part of the information processing apparatus 100.
The information generation apparatus 70 shown in FIG. 3 generates
the RGB.fwdarw.CMYK3D-LUT by correcting the calculation target
point for each of the six fundamental colors based on
RGB.fwdarw.Lab3D-LUT and CMYK.fwdarw.Lab4D-LUT. The
RGB.fwdarw.CMYK3D-LUT is a color conversion table for converting
the color image signals R, G and B of the three color RGB input
system into the color image signals C, M, Y, and K of the four
color CMYK output system. The information generation apparatus 70
is configured to have a memory 73, an operation section 74, a
control section 75, an image processing section 76, a ROM writer
77, and a display section 78.
To begin with, as a preliminary preparation for the generation of
the color conversion table, in a system of FIG. 1 configured to
have an RGB signal processing system 20, an image processing
apparatus 100, and a CMYK signal processing system 30, a color
patch image by converting a plurality of representative colors
defined in the Lab space into colors expressed by C, M, Y, K using
the CMYK.fwdarw.Lab4D-LUT is printed by the printer, this printed
color patch image is read out by the scanner in FIG. 1, and the
input data (to be described later) is obtained which is the data
indicating the plurality of colors defined in the Lab space by
converting each patch color using the RGB.fwdarw.Lab3D-LUT. This
input data is stored in the memory 73 of FIG. 3.
The input data D11 described above used for generating the
RGB.fwdarw.Lab3D-LUT or the CMYK.fwdarw.Lab4D-LUT, the Lab data
D12, the CMY data D13, the LCH data D14, etc., are stored in the
memory 73. The Lab values such as the lightness L*, the
chromaticity a* and b*, etc., are included in the Lab data D12. The
lightness L* and the chromaticity a* and b* are expressed by the
lightness-chromaticity coordinate space (hereinafter referred to as
the Lab color coordinates). The LCH values such as the lightness
L*, the chroma C*, the hue H*, etc., are included in the LCH data
D14. The lightness L*, the chroma C*, and the hue H* are expressed
by the lightness-hue pole coordinate system (hereinafter referred
to as the LCH color coordinate system).
The input data D11, Lab data D12, CMY data D13, LCH data D14, etc.,
are read out from the memory 73 into the RAM, etc., inside the
image processing section 76 based on the memory control signal S3.
The memory control signal S3 is outputted from the control section
75 to the memory 73. A hard disk drive or DRAMs are used for the
memory 73. In this example, the 8-bit input RGB values, is divided,
taking the example of 256 gradations, into 33 stages of 8
gradations each, and the values 0 to 32 are set for the respective
segments. The RGB value of the calculation target point of this
input data D11 is denoted by pin, and the output Lab value of the
Lab data D12 is denoted by qout.
The image processing section 76 is connected to the memory 73, and
the control section 75 is connected to the image processing section
76. The image processing section 76 is configured to include, for
example, a DSP (Digital Signal Processor) and a RAM, etc., and the
control section 75 is configured to have a CPU, a ROM, and a
RAM.
The operation section 74 and the display section 78 are connected
to the control section 75, and operation is made from the operation
section 74, for example, to select the gradation number equivalent
to each of the axes of RGB of the color three-dimensional
coordinate system obtained from within the RGB.fwdarw.Lab3D-LUT.
This selection operation is for setting the input RGB value of the
calculation reference point pc. The data set in the operation
section 74 becomes the operation data D3 and is outputted to the
control section 75. The three-dimensional color coordinate system,
etc., is displayed in the display section 78 based on the display
data D4. The display data D4 used is that stored as the graphic
display tool in the memory section provided in the control section
75 or in the image processing section 76.
The control section 75 corrects the calculation target point after
moving the positions of the maximum gradations of the six
fundamental colors (each of the colors R, Y, G, C, B, M) of the
four color CMYK output system to the position of the maximum
gradations of the six fundamental colors (each of the colors R, Y,
G, C, B, M) of the three color RGB input system, and also, corrects
by moving the positions other than that of the maximum gradation of
output system fundamental colors to the positions other than that
of the maximum gradation of input system fundamental colors and
corrects the calculation target point at each gradation of those
output system fundamental colors.
The control section 75 carries out, for example, in the LCH color
coordinate system, computes the first moving distance by obtaining
the difference between the position of the maximum gradation of the
output system fundamental colors and the position of the maximum
gradation of the input system fundamental colors, and also,
computes the second moving distance by obtaining the difference
between the position, outside the maximum gradation of the output
system fundamental colors and the position of the maximum gradation
outside the input system fundamental colors for each gradation of
that fundamental color. For example, the control system 75 is
handled to carry out the computation so that the output system
color image signals CMYK after converting the fundamental color
gradations of the input system color image signals RGB is identical
to that fundamental color gradation of the color gamut of that
color image signal of the output system, and it is possible to make
that calculation target point (coordinates value) become identical
for each fundamental color gradation of the output system color
image signals CMYK or of the input system color image signals
RGB.
Apart from this, at the time of computing the Lab CMYK value of the
Lab color coordinate system corresponding to the input RGB value of
the calculation target point pin based on the operation data D3
outputted from the operation section 74, the control section 75
outputs the input RGB value to the image processing section 76. For
example, the control section 75 sets the center RGB value in the
image processing section 76. In this example, we consider the case
when the center RGB value that becomes the calculation reference
point pc is set at the stage R=G=B=17 out of the 33 stages of
lattice points. The setting of the center RGB value need not be set
at the 17.sup.th stage but can also be set at any other stage. The
input RGB value of this calculation reference point is taken as pc
and its Lab value is taken as qc.
The image processing section 76 described above carries out the
color gamut surface search processing under the operation from the
operation section 74 and the control of the control section 75. In
addition, the image processing section 76 inputs the input data
D11, and executes the triangle setting processing, the triangular
pyramid setting processing, the inclusion judgment processing, and
the inside or outside color gamut judgment processing to be
described later.
The RGB.fwdarw.CMYK3D-LUT generation control is carried out based
on the detection result of the inside or outside color gamut
judgment processing obtained from the image processing section 76.
At this time, the control section 75 executes the internal
insertion processing mode when the input RGB value of the
calculation target point pin detected by the image processing
section 76 is inside the printer color gamut, and executes the
external insertion processing mode when the input RGB value of the
calculation target point pin detected by the image processing
section 76 is outside the printer color gamut. In the external
insertion processing mode, the control section 75 carries out
external insertion interpolation by calculating the Lab CMYK value
in the Lab color coordinates system corresponding to the input RGB
value of the calculation target point pin. Of course, it is not
necessary to restrict to external insertion processing but also
compression processing can be made.
The ROM writer 77 is connected to the control section 75 and to the
image processing section 76, and operates to write the
RGB.fwdarw.CMYK3D-LUT in the ROM based on the ROM write signal S4
and the ROM data Dout. The ROM data Dout is the data for
configuring the RGB.fwdarw.CMYK3D-LUT. This RGB.fwdarw.CMYK3D-LUT
written in the ROM has been improved compared to the conventional
examples. The ROM writing signal S4 is outputted from the control
section 75 to the ROM writer 77. A judgment is made as to whether
or not this Lab value is inside the color gamut or outside the
color gamut of the corresponding CMY.fwdarw.Lab3D-LUT (inside or
outside color range judgment processing).
In this example, when the calculation target point pin is judged to
be outside the printer color gamut by the inside or outside color
range judgment processing, the image processing section 76 is
controlled by the control section 75 and executes the compression
processing.
FIG. 4 is a flow chart (main routine), which is executed in the
information generation apparatus 70, showing an example of
generating the RGB.fwdarw.CMYK3D-LUT with the calculation target
points compensated for each of the six fundamental colors. FIG. 5
is a flow chart (subroutine) showing an example of generating the
CMY.fwdarw.CMYK3D-LUT using a GCR algorithm, and FIG. 6 is a flow
chart (subroutine) showing an example of generating the
CMY.fwdarw.Lab3D-LUT.
In this preferred embodiment, the assumption is the case of
generating the color conversion table for color conversion of the
color image signals R, G, and B of the three color RGB input system
into the color image signals C, M, Y, and K of the four color CMYK
output system. At this time, not only the calculation target point
of fundamental color points but also the calculation target points
of fundamental color gradations are moved. In this example, the
case is considered that the input data D11 and the Lab data D12 for
generating the RGB.fwdarw.Lab3D-LUT and also the CMY data D13 for
generating the CMYK.fwdarw.Lab4D-LUT are stored beforehand in the
memory 73.
In this example, the case is considered of carrying out correction
of the calculation target point by moving the positions of the
maximum gradations of the six fundamental colors (each of the
colors R, Y, G, C, B, M) of the four color CMYK output system to
the position of the maximum gradations of the six fundamental
colors (each of the colors R, Y, G, C, B, M) of the three color RGB
input system, and thereafter, corrects by moving the positions
other than that of the maximum gradation of output system
fundamental colors to the positions other than that of the maximum
gradation of input system fundamental colors and corrects the
calculation target point at each gradation of that output system
fundamental colors.
Further, as a result of carrying out the inside or outside color
gamut judgment, the operation moves to the internal insertion
processing mode only when the calculation target point is judged to
be inside the printer color gamut, and moves to the external
insertion processing mode if it is judged to be outside the printer
color gamut.
Taking these as the condition for preparation of tables, under the
instruction of the control section 75 in Step A1 of the flow chart
shown in FIG. 4, the input data D11 and the Lab data D12 are read
from the memory 73 into the image processing section 76 and are
expanded in the RAM, etc. In the RAM, the input data D11 and the
Lab data D12 are expanded and the RGB.fwdarw.Lab3D-LUT is
generated. The RGB.fwdarw.Lab3D-LUT is generated using, for
example, the image processing method disclosed in Japanese Patent
Public Disclosure No. 3174604. Some other methods can be used for
generating this table, and it is sufficient if the
RGB.fwdarw.Lab3D-LUT is prepared. In this example, the number of
lattice points is 18.times.18.times.18 (=5832).
Next, under instruction from the control section 75 in Step A2, the
CMY data D13 is read from the memory 73 into the image processing
section 76 and is expanded in the RAM. The CMY data D13 is expanded
in the RAM and the CMYK.fwdarw.Lab4D-LUT is generated. The
CMYK.fwdarw.Lab4D-LUT is generated using, for example, the image
processing method disclosed in Japanese Patent Application Laid
Open to Public Inspection No. H06-242523. Some other methods can be
used for generating this table, and it is sufficient if the
CMYK.fwdarw.Lab4D-LUT is prepared. In this example, the number of
lattice points is 17.times.17.times.17.times.17 (=83521).
Next, the operation moves on to Step A3, and the image processing
section 76 generates the CMY.fwdarw.Lab3D-LUT from the
CMYK.fwdarw.Lab4D-LUT based on a GCR algorithm. At this time, as
the number of lattice points of the CMY.fwdarw.Lab3D-LUT, for
example, set about 33.times.33.times.33 (=35,937) points. In order
to generate a CMY.fwdarw.Lab3D-LUT with such a number of lattice
points, the execution is carried out by dividing the process into
the two stages of the processing of generating the
CMY.fwdarw.CMYK3D-LUT using a GCR algorithm, and the processing of
generating the CMY.fwdarw.Lab3D-LUT by triangular pyramid
interpolation using the CMYK.fwdarw.Lab4D-LUT.
Processing of Preparation of CMY.fwdarw.CMYK3D-LUT:
This processing is executed by the image processing section 76. For
example, using a GCR algorithm, in order to generate the
CMY.fwdarw.CMYK3D-LUT, the subroutine shown in FIG. 5 is called and
the CMY value is set in its Step B1. At this time, the C, M, and Y
values respectively are stepped up from 0 to 32, and the operation
of computing the CMYK value at the respective CMY values is
repeated successively.
Further, the operation moves on to Step B2 and the minimum value is
outputted from the CMY values set earlier. For example, if the
computed values are C=10, M=12, and Y=7, the minimum value (min) is
min=7. Thereafter, the operation moves on to Step B3 and the black
color value K is computed. In this example, the black color value K
is obtained by multiplying the above mentioned value min by GCR
(UCR: Under Color Removal), that is, it is obtained using Equation
(1) below. K=min.times.GCR(UCR) (1)
(Generation of K)
This GCR indicates the extent of generating the black color value
K, and a value in the range 0.0 to 1.0 is selected for this
parameter. For example, when GCR is 0.9, K=7.times.0.9=6.3. In this
example, if the value of K is large, the amount of black color used
becomes larger, and if the value of K is small, the amount of black
color used becomes smaller.
Next, in Step B4, the new CMY value is computed. In this example,
the new C, M, and Y values are computed by deducting the above
value K from the original C, M, and Y values. For example, if the
new C, M, and Y values are denoted respectively as newC, newM, and
newY, they can be obtained by the following Equation (2).
newC=C-K.fwdarw.newC=10-6.3=3.7 newM=M-K.fwdarw.newM=12-6.3=5.7
newY=Y-K.fwdarw.newY=7-6.3=0.7 (2)
In this manner, the CMYK value corresponding to a particular CMY
value is computed. Thereafter, the operation moves on to Step B5,
and a judgment is made as to whether or not the CMYK values have
been computed for the entire CMY data D13 (end judgment). If the
computation of the CMYK values for the entire CMY data D13 has not
been completed, the operation returns to Step B1 and the steps are
repeated. When the CMYK values have been computed for all the CMY
data D13, the CMY.fwdarw.CMYK3D-LUT is generated.
Processing of Preparation of CMY.fwdarw.Lab3D-LUT:
This processing is executed by the image processing section 76.
This CMY.fwdarw.Lab3D-LUT is generated by triangular pyramid
interpolation using the CMYK.fwdarw.Lab4D-LUT. For example, each of
the CMYK values of the CMY.fwdarw.CMYK3D-LUT generated in the Steps
B1 to B5 of the subroutine described above is converted into a Lab
value using CMYK.fwdarw.Lab4D-LUT, and the CMY.fwdarw.Lab3D-LUT is
generated by repeating this conversion processing.
For example, using the GCR algorithm, in order to prepare the
CMY.fwdarw.Lab3D-LUT, the subroutine shown in FIG. 6 is called and
the CMY value is set in its Step C1. In this example, regarding the
CMY.fwdarw.CMYK3D-LUT generated earlier, when the CMYK values are
C=135, M=232, Y=96, and K=8, referring to K=8, from the
CMYK.fwdarw.Lab4D-LUT, the CMYK (K=first).fwdarw.Lab3D-LUT and CMYK
(K=second).fwdarw.Lab3D-LUT are used.
Further, in Step C2, the Lab value is computed by carrying out
triangular pyramid interpolation with two K values using the
CMYK.fwdarw.Lab4D-LUT. For example, the Lab value corresponding to
the address C=135, M=232, and Y=96 in the CMYK
(K=first).fwdarw.Lab3D-LUT is computed by triangular pyramid
interpolation processing. As a result of this triangular pyramid
interpolation processing, the Lab value of K=first (hereinafter
referred to as Lab1) will be Lab1=[L, a, b]=[35, 37, -4].
Further, the Lab value corresponding to the address C=135, M=232,
and Y=96 in the CMYK (K=second).fwdarw.Lab3D-LUT is computed by
triangular pyramid interpolation processing. As a result of this
triangular pyramid interpolation processing, the Lab value of
K=second (hereinafter referred to as Lab2) will be Lab2=[L, a,
b]=[34, 36, -4].
Thereafter, in Step C3, linear interpolation is made for the two
values Lab1 and Lab2 obtained earlier based on the black color
value K. Here, if we denote the new Lab value in the case when K=8
by newLab, it can be expressed by the following Equation (3).
newLab=((16-K(=8))/16).times.Lab1+((16-K(=8))/16).times.Lab2
(3)
When newLab is computed by substituting the values of Lab1 and Lab2
in Equation (3), it will be newLab=[L, a, b]=[34, 37, -4]. The
value newLab obtained in this manner becomes the Lab value
corresponding to the CMYK value for the K value.
After that, the operation moves on to Step C4, and a judgment is
made as to whether or not the Lab values have been computed for the
CMYK values at the K value of the entire CMY data D13 (end
judgment). If the computation of the Lab values for the CMYK values
at the K value of the entire CMY data D13 has not been completed,
the operation returns to Step C1 and the above processing is
repeated. When the Lab values for the CMYK values at the K value of
all the CMY data D13 have been computed and the processing has been
repeated for all the lattice points, it is possible to generate the
CMY.fwdarw.Lab3D-LUT. When the CMY.fwdarw.CMYK3D-LUT and the
CMY.fwdarw.Lab3D-LUT have been generated using the above two
subroutines, the processing returns to Step A3 of the main
routine.
After that, the operation moves on to Step A4, and the processing
of modifying the CMY.fwdarw.Lab3D-LUT by fundamental color
adjustment is carried out. For example, in Step A41, to begin with,
the adjustment processing of fundamental color points is executed.
After that, in Step A42, the adjustment processing of fundamental
color gradations is executed.
Adjustment Processing of Fundamental Color Points:
FIG. 7 is a diagram showing an example of the relationship between
the input side fundamental color points of the CMYRGB colors in the
Lab coordinate system and the printer side fundamental color points
(Max). In this example, the first moving distance is computed by
obtaining the differences between the positions of the maximum
gradation of the output side (printer) fundamental colors and the
positions of the maximum gradation of the input side fundamental
colors. The horizontal axis in FIG. 7 is the color saturation a* of
the color Y or of the color B, and the vertical axis is the color
saturation b* of the color G or of the color M. The filled circles
in the figure indicate the fundamental color gradation (Max) on the
printer side, and the open circles indicate the fundamental color
gradation (Max) on the input side. In this figure, the difference
between the filled circles of the printer side fundamental color
gradation (Max) and the open circles of the input side fundamental
color gradation (Max) is the error (DIFF). The method used in the
present preferred embodiment is that, regarding the colors CMYRGB,
the calculation target points of the fundamental color points are
moved from the output side to the input side, and also, in other
cases the calculation target points are moved from the output side
to the input side for each fundamental color gradation.
FIG. 8 is a conceptual diagram showing an example of adjustment of
the fundamental color points in the Lab coordinate system. When the
position of the new calculation target point shown in FIG. 8 is
taken as New, it is obtained by two processings #I and #II. In
processing #I, from the relationship between the color R of the
printer and the color R of the input, the error between these input
value and output value is obtained (the vector Diff Red), and also,
from the relationship between the color M of the printer and the
color M of the input, the error between these input value and
output value is obtained (the vector DiffMagenta). Linear
interpolation is carried out for these two vectors using Hue.
Next, in processing #II, the Diff amount of the position New of the
new calculation target point is obtained based on the chroma C* and
the maximum chroma MaxC* of its Hue. For example, Diff(C*/MaxC*) is
computed. The respective NewL*, NewC*, and NewH* are calculated. By
eliminating the difference (Diff) between the two, it is possible
to match the positions of the Max gradations of the six fundamental
colors of the input side and of the output side.
If the lightness L*, chroma C*, and hue H* of the position New of
the new calculation target point are obtained in this manner, it is
possible to move the calculation target points of the fundamental
color points from the output side to the input side. Further, as is
shown in FIG. 8, the adjustment of the six fundamental color points
in the Lab coordinate system is carried out, for the point having
the input C* of MaxC*, by the same amount as that of the obtained
DiffL*, C*, and H*. The amount of adjustment becomes smaller as the
origin is approached by processing using C*/MaxC*.
FIG. 9 is a flow chart (subroutine) showing an example of adjusting
the positions of the fundamental color points. FIG. 10(A), FIG.
10(B), and FIG. 11 are diagrams for supplementing this. FIG. 10(A)
and FIG. 10(B) are diagrams showing an example of the relationship
between the lightness error DiffL* and the hue Hue, and an example
of the relationship between its chroma error DiffC* and hue Hue.
FIG. 11 is a diagram showing an example of the relationship between
the error in its hue DiffH* and its hue Hue.
In this example, the input color gamut generated in Step A3 shown
in FIG. 4 above is taken as RGB.fwdarw.Lab3D-LUT, and output color
gamut is taken as CMY.fwdarw.Lab3D-LUT. Before entering the flow
chart shown in FIG. 9, the Lab values are converted to the LCH
coordinate system using the formula (L*=L*, c*=(a*.sup.2+
b*.sup.2).sup.1/2, H*=arc tan(b*/a*)).
After these conversions, in Step E1, the processing of setting the
color difference (DIFF) of the six fundamental color points is
carried out. At this time, the color difference (DIFF) in the LCH
coordinate system of the fundamental color points between the input
color gamut and the output color gamut is calculated. For example,
taking the lightness of red color of the input color gamut as
InputGamutRedL*, the lightness of red color of the output color
gamut as OutputGamutRedL*, and the error in the lightness of red
color as DiffRedL*, the relationship between them is given by
Equation (4) below. DiffRedL*=InputGamutRedL*-OutputGamutRedL*
(4)
Further, taking the degree of saturation (chroma) of red color of
the input color gamut as InputGamutRedC*, the degree of saturation
(chroma) of red color of the output color gamut as
OutputGamutRedC*, and error in the degree of color saturation of
red color as DiffRedC*, the relationship between them is given by
Equation (5) below. DiffRedC*=InputGamutRedC*-OutputGamutRedC*
(5)
Further, taking the color phase (hue) of red color of the input
color gamut as InputGamutRedH*, the color phase (hue) of red color
of the output color gamut as OutputGamutRedH*, and error in the
color phase (hue) of red color as DiffRedH*, the relationship
between them is given by Equation (6) below.
DiffRedH*=InputGamutRedH*-OutputGamutRedH* (6)
The amount of error (Diff) is obtained by computing these Equations
(4) to (6). The other fundamental colors of Y, G, C, B, and M are
obtained in a similar manner.
Graphs such as those shown in FIG. 10(A), FIG. 10(B), and FIG. 11
are obtained when the error in lightness DiffL* of each color of
the six fundamental colors, namely the colors R, Y, G, C, B, M, the
error in their chroma DiffC*, and the error in their color phase
(hue) DiffH* are plotted along the vertical axis versus the hue
(Hue, 0 to 360.degree.) along the horizontal axis.
Next, in Step E2 the LCH value to calculate is set. For example,
the LCH value (L*, C*, H*) of the output color gamut is set for one
point at a time and this is taken as the input value. When
calculation target points are present for 33.sup.3 lattice points,
the loop process indicated below has to be repeated 33.sup.3
times.
Further, in Step E3, a judgment process based on the hue is
executed. For example, in the drawings shown in FIG. 10(A), FIG.
10(B), and FIG. 11, a judgment is made as to between which of
DiffL*, DiffC*, and DiffH* is the hue of the input value, and the
amount of DIFF (DiffL*, C*, H*) at the input hue and the maximum
chroma (MaxC*) at the input hue are obtained by linear
interpolation. This is for making the maximum positions of the six
fundamental colors identical.
For example, when the hue of the input value is 65.degree., the
lightness error DiffL* for Hue=65.degree. is read out from FIG.
10(A), the chroma error DiffC* for Hue=65.degree. is read out from
FIG. 10(B), and the chroma error DiffH* for Hue=65.degree. is read
out from FIG. 11. Next, in Step E4, the processing of computing the
moving distance is executed. At this time, the LCH value at the new
calculation target position is obtained from the input value and
the Diff value. Here, if the new lightness L*, the new chroma C*,
and the new hue H* are denoted by NewL*, NewC*, and NewH*,
respectively, and if the DIFF amounts of lightness L*, chroma C*,
and hue H* are denoted by Diff (C*/MaxC*), Diff(C*/MaxC*), and
DiffH*, the values are computed by evaluating the following
Equation (7). NewL*=L*+Diff(C*/MaxC*) NewC*=C*+Diff(C*/MaxC*)
NewH*=H*+DiffH* (7)
Thereafter, an end judgment is carried out in Step E5. In this
example, a judgment is made as to whether or not the computation
(adjustment) of moving distance has been made for all the 33.sup.3
input values. If these computation operations have not been
competed, the operation returns to Step E2 and the processing of
setting the LCH value is repeated. Further, the color gamut of the
33.sup.3 points of LCH data D14 that have been subjected to
computation processing is converted and returned to the Lab
coordinate system using the formula [L*=L*, a*=cC*cos(H*/180.pi.),
b*=sin(H*/180.pi.)]. Because of this, it is possible to make
identical the positions of maximum gradations of the six
fundamental colors of the input side and output side.
Example of Adjustment of Fundamental Color Gradation Positions:
FIG. 12 is a conceptual diagram showing an example of adjusting the
fundamental color gradation position in the Lab coordinate system.
The horizontal axis in FIG. 12 indicates the color saturation a* of
the color M, and the vertical axis indicates the color saturation
b* of the color Y. This coordinate system indicates the input value
and the output value of the color R, and is one in which the LCH
coordinates (pole coordinates) expressing the Chroma vector and the
Hue at the origin of the Lab coordinate system is superimposed.
In this coordinate system, at the origin of the Lab coordinates,
the input value InputGamutRed1 of the first gradation of the color
R on the input side matches with the output value OutputGamutRed1
of the first gradation of the color R on the output side, and also,
at the position of the maximum gradation, the input value
InputGamutRed18 of the eighteenth gradation of the color R on the
input side matches with the output value OutputGamutRed33 of the
maximum gradation of the color R on the output side.
In this example, the output color gamut of the output values
OutputGamutRed2 to OutputGamutRed32 of the 2.sup.nd to 32.sup.nd
gradations of the color R on the output side is moved to the input
color gamut of the input values InputGamutRed2 to InputGamut17 of
the 2.sup.nd to 17.sup.th gradations of the color R on the input
side. The moving distance is obtained based on the chroma C* vector
and the hue Hue in the LCH coordinate system.
In the figure, DiffRed3 is the error between the output value
OutputGamutRed2 of the third gradation of the color R and the input
value ModfRed3 of the third gradation of the color R of the
calculation target point that is to be moved.
FIG. 13 is a flow chart (subroutine) showing an example of
processing at the time of adjusting the fundamental color gradation
position. In this example, the second moving distance is computed
by obtaining the difference between the position other than that of
the maximum gradation of the fundamental colors of the output
system and the position other than that of the maximum gradation of
the fundamental colors of the input system for each gradation of
that fundamental color. Even in this case, the output color gamut
after the fundamental color point has been adjusted is taken as
CMY.fwdarw.Lab3D-LUT (33.sup.3 points), and the input color gamut
is taken as RGB.fwdarw.Lab3D-LUT (18.sup.3 points). Further, before
entering the flow chart, the Lab values are converted to the LCH
coordinate system using the formula [L*=L*, c*=(a*.sup.2+
b*.sup.2).sup.1/2, H*=arc tan(b*/a*)].
Next, in Step F1 of the flow chart shown in FIG. 12, the processing
of setting the six fundamental color gradations is executed. For
example, the LCH values of the fundamental color gradations are
extracted from the input color gamut. The input color gamut can be
a color gamut with 18.sup.3 points. If the input color gamut is a
Red color gamut, 18 gradations will have to be extracted. Here, the
lightness of the input color gamut of the first gradation of red
color is InputGamutRed1L*, its chroma is taken as InputGamutRed1C*,
and its hue is taken as InputGamutRed1H*. In the following, for the
2.sup.nd to the 18.sup.th gradations the setting is made as
InputGamutRed2L*, InputGamutRed2C*, InputGamutRed2H*, . . . .
InputGamutRed18L*, InputGamutRed18C*, InputGamutRed18H*. Similar
settings are also made for the other colors Y, G, C, B, and M.
In addition, in the LCH coordinate system, the LCH values of the
fundamental color gradations are extracted from the output color
gamut. In the case of a color gamut with 33.sup.3 points, if it is
a color R gradation, 33 gradations will have to be extracted. Here,
the lightness of the output color gamut of the first gradation of
red color is OutputGamutRed1L*, its chroma is taken as
OutputGamutRed1C*, and its hue is taken as OutputGamutRed1H*. In
the following, for the 2.sup.nd to the 33.sup.rd gradations the
setting is made as OutputGamutRed2L*, InputGamutRed2C*,
OutputGamutRed2H*, . . . OutputGamutRed33L*, OutputGamutRed33C*,
OutputGamutRed33H*. Similar settings are also made for the other
colors Y, G, C, B, and M.
Next, in Step F2, the computation of the error (Diff) in the six
fundamental color gradations is carried out. In this example, the
L*, C*, H*, and Diff for 33 gradations and also for six fundamental
colors is computed using linear interpolation. Here, the error in
the lightness of the first gradation of the color R is taken as
DiffRed1L*, its chroma error as DiffRed1C*, and its hue error as
DiffRed1H*. Thereafter, even for the 2.sup.nd to 33.sup.rd
gradations, settings are made as DiffRed2L*, DiffRed2C*,
DiffRed2H*, . . . , DiffRed33L*, DiffRed33C*, DiffRed33H*. Settings
are made in a similar manner ever for the other fundamental colors
of Y, G, C, B, and M. As is shown in FIG. 14(A), FIG. 14(B), and
FIG. 15, each of these has been plotted taking the Chroma along the
X-axis, and each of the values L*, C*, and H* (0 to 100) along the
Y-axis (vertical axis), and the input values of color R
InputGamutRed1 to InputGamutRed18 are connected by straight
lines.
To the chroma along the horizontal axis shown in FIG. 14(A), for
example, the output value of the third gradation of the color R
OutputGamutRed3C* is inputted. The lightness L* at the point where
this output value OutputGamutRed3C* meets the straight line becomes
the modifRed3L* to which the OutputGamutRed3 has to be moved.
In a similar manner, to the chroma along the horizontal axis shown
in FIG. 14(B), the output value of the third gradation of the color
R OutputGamutRed3C* is inputted. The chroma C* at the point where
this output value OutputGamutRed3C* meets the straight line becomes
the modifRed3C* to which the OutputGamutRed3 has to be moved.
Further, to the chroma along the horizontal axis shown in FIG. 15,
the output value of the third gradation of the color R
OutputGamutRed3C* is inputted. The hue Hue* at the point where this
output value OutputGamutRed3C* meets the straight line becomes the
modifRed3Hue* to which the OutputGamutRed3 has to be moved. The
lightness L*, chroma C*, and hue H* are computed by linear
interpolation. With this, the L*, C*, and H* to be moved are
obtained, and next, the moving distance of each gradation of the
fundamental colors (the second moving distance) is computed. For
example, these are obtained by Equation (8) below taking the
lightness of the input value of the third gradation of the color R
as InputGamutRed3L*, the lightness of the output value of the third
gradation of the color R as OutputGamutRed3L*, and the error in the
lightness of the third gradation of the color R as DiffRed3L*.
DiffRed3L*=InputGamutRed3L*-OutputGamutRed3L* (8)
Further, these are obtained by Equation (9) below taking the chroma
of the input value of the third gradation of the color R as
InputGamutRed3C*, the chroma of the output value of the third
gradation of the color R as OutputGamutRed3C*, and the error in the
chroma of the third gradation of the color R as DiffRed3C*.
DiffRed3C*=InputGamutRed3C*-OutputGamutRed3C* (9)
This value will always be "0".
In addition, these are obtained by Equation (10) below taking the
hue of the input value of the third gradation of the color R as
InputGamutRed3H*, the hue of the output value of the third
gradation of the color R as OutputGamutRed3H*, and the error in the
hue of the third gradation of the color R as DiffRed3H*.
DiffRed3H*=InputGamutRed3H*-OutputGamutRed3H* (10)
When this is repeated for all the six fundamental colors and all
the 33 gradations, the plot shown in FIG. 16 is obtained in which
the horizontal axis corresponds to the hue Hue and the vertical
axis corresponds to the chroma C*. In this figure, 6.times.33
lattice points have been plotted, correspondence is established
with the data indicating the amounts of movement for the respective
points, and these data items are stored in the memory.
Next, the LCH values to calculate are set in Step F3. At this time,
the LCH value (L*, C*, H*) of the output color gamut is set for one
point at a time and this is taken as the input value. When the
calculation target points are present for 33.sup.3 lattice points,
the loop process is repeated 33.sup.3 times.
Next, in Step F4, the processing of judging inclusion in triangular
shape is executed. At this time, in the hue Hue-chroma C* plane as
is shown in FIG. 16, when the chroma C* and the hue H* are
inputted, a judgment is made as to between which lattice points is
the calculation target point rin is included. In FIG. 16, the
horizontal axis corresponds to the hue Hue (-30.degree. to
330.degree.) and the vertical axis corresponds to the chroma (0 to
100%). The three lattice points in the figure indicate the part for
executing the inclusion judgment in triangular shapes. The
processing of inclusion judgment in triangular shapes is done by
searching in the direction of the arrow in the figure. The judgment
is made by successively setting the three lattice points. At this
time, the lightness L*, chroma C*, and the hue H* that have been
inputted are taken as the lightness rinL*, chroma rinC*, and the
hue rinH* of the calculation target point rin.
Further, the position r of the lightness L*, chroma C*, and the hue
H* of the three lattice points is taken respectively as r1L, r1C,
r1H, r2L, r2C, r2H, r3L, r3C, and r3H, and the moving distance s of
the three lattice points will be s1L, S1C, s1H, s2L, s2C, s2H, s3L,
s3C, and s3H when expressed in the s coordinate system.
At this time, the c*H* coordinate system using the chroma c* and
hue H* at the position r is set as shown in FIG. 17(A), if the
vector between the positions r2-r1 and the vector between the
positions r3-r1 are denoted respectively by Y.sub.0 and Y.sub.1, it
is possible to express the vectors between the positions rin-r1
using Y.sub.0 and Y.sub.1 by the Equation (11) below.
rinC-r1C=Y.sub.0(r2C-r1C)+Y.sub.1(r3C-r1C)
rinH-r1H=Y.sub.0(r2H-r1H)+Y.sub.1(r3H-r1H) (11)
The values of Y.sub.0 and Y.sub.1 are derived (obtained) from this
relationship. If Y.sub.0 and Y.sub.1 are satisfying the following
conditions, then it is judged that the calculation target point rin
is included in that color gamut. These conditions are taken as
Y.sub.0+Y.sub.1<1.0, Y.sub.0>0, Y.sub.1>0.
Next, in Step F6, the processing of computing the moving distance
is executed. In this example, a similar equation is also valid in
the triangular interpolation shown in FIG. 17(B) (coordinate system
of the moving distance: s coordinate system). As has been shown in
FIG. 17(B), if the vector of the moving distance between s2-s1 and
the vector of the moving distance between s3-s1 are denoted
respectively by Y.sub.0 and Y.sub.1, and next, the vector of the
moving distance between sout-s1 can be expressed by the Equation
(12) given below using Y.sub.0 and Y.sub.1, taking the moving
distance Sout of the calculation target point rin to be SoutL,
SoutC, and SoutH respectively for the lightness L*, chroma C*, and
hue H*. SoutL=Y.sub.0(r2L-r1L)+Y.sub.1(r3L-r1L)+s1L
SoutC=Y.sub.0(r2C-r1C)+Y.sub.1(r3C-r1C)+s1C
SoutH=Y.sub.0(r2H-r1H)+Y.sub.1(r3H-r1H)+s1H (12)
The new LCH value is obtained from these SoutL, SoutC, and SoutH.
Here, if the new LCH values are denoted by NewL*, NewC*, and NewH*,
they can be expressed by the following Equation (13).
NewL*=L*+SoutL NewC*=C*+SoutC NewH*=H*+SoutH (13)
After that, in Step F7, the end judgment processing is executed.
Here, the judgment is made as to whether the processing of
adjusting the positions for all the gradations of all the
fundamental colors for the 33.sup.3 input values. If these
adjustment processings have not been completed, the operation
returns to the step of setting the LCH value to calculate and the
above processing is repeated. If the adjustment processings have
been completed, the color gamut of the 33.sup.3 LCH data D14 for
which adjustment processing has been made is converted using the
formula [L*=L*, a*=C*cos(H*/180.pi.), b*=sin(H*/180.pi.)] and
returned to the Lab coordinate system.
With this, it is possible to superimpose the color gamut of the
fundamental color gradations of the input side shown in FIG. 18
with the color gamut of the fundamental color gradations after
adjustment on the output side. In FIG. 18, the horizontal axis is
the chroma a* of the color M, and the vertical axis is the chroma
b* of the color Y. This has been obtained by moving the output side
fundamental color gradations over to the input side fundamental
color gradations. The filled circles in the figure denote the input
side fundamental color gradations and the open circles indicate the
output side fundamental color gradations.
Next, the operation returns to Step A42 shown in FIG. 4, and after
that, the operation moves to Step A5, and the Lab values to
calculate are set in the Lab coordinate system. At this time, the
CMY values corresponding to each of the Lab values of the
RGB.fwdarw.Lab3D-LUT are computed from the CMY.fwdarw.Lab3D-LUT.
The calculation target point in this Lab coordinate system is taken
as pin.
Next, the operation moves on to Step A6, the Lab value set (input)
earlier is searched from CMY.fwdarw.Lab3D-LUT, and a judgment is
made as to whether this Lab value is inside or outside the color
gamut of the corresponding CMY.fwdarw.Lab3D-LUT (inside or outside
color gamut judgment processing). Here, at first, regarding the CMY
values of the CMY.fwdarw.Lab3D-LUT, the center value of that
calculation reference point, which is the 16.sup.th Lab value of
C=M=Y, is taken as pc, and the three points that form the vertices
of the triangle in the surface of that CMY.fwdarw.Lab3D-LUT are set
successively (triangle setting processing). In this example, the
surface that becomes the smallest unit in the color gamut surface
is a triangle formed by three input data D1.
The Lab values of the three vertex points are taken to be p1, p2,
and p3, respectively. In a similar manner, the CMY values of the
addresses of this calculation reference point and the vertices are
taken to be qc, q1, q2, and q3. Also, X.sub.0, X.sub.1, and X.sub.2
are, for example, the weighting factors in the Lab coordinate
system set for the calculation reference point taking the center
value pc of the Lab values. This example is a case in which the
calculation target point with a Lab value equal to pc has been
assigned at a position at which it penetrates the triangle with the
Lab values of its vertices being p1, p2, and p3.
Here, regarding the Lab values of CMY.fwdarw.Lab3D-LUT, when its
center value of C=M=Y=16.sup.th Lab value is taken as the
calculation reference point pc, the calculation target point is
taken as pin, the Lab values of the three vertices set on the
surface of this CMY.fwdarw.Lab3D-LUT are taken as p1, p2, and p3,
and the weighting factors in the Lab coordinate system set for the
calculation reference point pc are taken as X.sub.0, X.sub.1, and
X.sub.2, the relationship between the calculation reference point
pc and the calculation target point pin is given by the following
Equation (14).
.times..times..times..times..times..times..times..times..times.
##EQU00001##
FIG. 19 is a diagram showing an example of color gamut surface
search of the calculation target point pin in the Lab coordinate
system. According to the example of the color gamut surface search
shown in FIG. 19, a search is made of which color gamut surface in
the color gamut surface of the input data D11 does the straight
line connecting the input RGB value of the calculation target point
pin and the center RGB value of the calculation reference point pc
intersect. For example, the Lab value set (input) in the image
processing section 76 is searched from the
CMY.fwdarw.Lab3D-LUT.
This equation of relationship is solved and the weighting factors
X.sub.0, X.sub.1, and X.sub.2 are obtained. At this time, if
X.sub.0>0, X.sub.1>0, and X.sub.2>0, and also,
(X.sub.0+X.sub.1+X.sub.2)<1, it implies that the calculation
target point is within the printer color gamut. Further, if
X.sub.0>0, X.sub.1>0, and X.sub.2>0, and also,
(X.sub.0+X.sub.1+X.sub.2).gtoreq.1, it implies that the calculation
target point is outside the printer color gamut.
As a result of carrying out the process of judging inside or
outside color gamut described above, if the calculation target
point pin is judged to be outside the printer color gamut, the
operation moves on to Step A6 in which, using CMY.fwdarw.Lab3D-LUT,
for example, using the external insertion processing mode that
utilizes the following Equation (15) and its expanded equation, the
CMY value (qout) corresponding to the input Lab value is computed
(external insertion processing mode).
Here, the Equation (15) used in the external insertion processing
mode is explained. Equation (15) is obtained by the color gamut
surface search processing in the CMY coordinate system. FIG. 20 is
a diagram showing an example of color gamut surface search in the
CMY coordinate system. According to the example of color gamut
surface search, the output CMY value of the calculation reference
point being pc in the CMY coordinate system, and the CMY values of
the different vertices being q1, q2, and q3, the external insertion
processing mode computes the CMY value (qout) corresponding to the
input Lab values.
Here, regarding the CMY values of the CMY.fwdarw.Lab3D-LUT, when
the CMY value of that calculation target point is taken as qout,
and the CMY values of the addresses of the lattice points set on
the surface of that CMY.fwdarw.Lab3D-LUT are taken to be q1, q2,
and q3, the CMY value of the address set at the calculation
reference point is taken as qc, and the weighting factors in the
Lab coordinate system set for the calculation reference point pc
are taken as X.sub.0, X.sub.1, and X.sub.2, the value of qout is
obtained by the following Equation (15).
.times..times..times..times..times..times..times..times..times.
##EQU00002##
Expanding this equation, we get:
qout.sub.--C=(q1Cqc.sub.--C).times.X.sub.0+(q2.sub.--Cqc.sub.--C).times.X-
.sub.1+(q3.sub.--C-qc.sub.--C).times.X.sub.2+qc.sub.--C
qout.sub.--M=(q1.sub.--M-qc.sub.--M).times.X.sub.0+(q2.sub.--M-qc.sub.--M-
).times.X.sub.1+(q3.sub.--M-qc.sub.--M).times.X.sub.2+qc.sub.--M
qout.sub.--Y=(q1.sub.--Y-qc.sub.--Y).times.X.sub.0+(q2.sub.--Y-qc.sub.--Y-
).times.X.sub.1+(q3.sub.--Y-qc.sub.--Y).times.X.sub.2+qc.sub.--Y
Negative values are obtained at the time of computing this Equation
(15). Further, it is possible to evaluate this Equation (15)
beforehand and store the results in the ROM of the image processing
section 76.
As a result of carrying out the process of judging inside or
outside color gamut described above, if the calculation target
point pin is judged to be inside the printer color gamut, the
operation moves on to Step A7 in which, using the Lab value set
earlier, the CMY value is searched by inclusion judgment within the
CMY.fwdarw.Lab3D-LUT, and the CMY value corresponding to this
inclusion judgment is computed. In order carry out this inclusion
judgment, the Lab values of four points (points p4, p5, p6, and p7)
are set successively from inside the CMY.fwdarw.Lab3D-LUT. In a
similar manner, the CMY values of four points (points q4, q5, q6,
and q7) are set (Triangular pyramid setting process).
Here, the inclusion judgment in the Lab coordinate system is
explained. FIG. 21 is a diagram showing an example of inclusion
judgment in the Lab coordinate system. According the inclusion
judgment shown in FIG. 21, the judgment is made as to whether or
not the input RGB value of the calculation target point pin is
included in the range of the plot of input RGB values (inclusion
judgment processing). In this example, in the 5.sup.3 points of
input data D11, the number of lattice points with the smallest unit
of volume, is four lattice points forming a triangular pyramid. In
this example, the triangular pyramid setting process is executed,
and triangular pyramids are set successively from among a plurality
of triangular pyramids. The Lab values of the lattice points of the
triangular pyramid set here is taken as p4, p5, p6, and p7. Also,
Y.sub.0, Y.sub.1, and Y.sub.2 are, for example, the weighting
factors in the Lab coordinate system set for the calculation
reference point taking the center value pc of the Lab values. This
example is a case in which the calculation target point with a Lab
value equal to pin has been assigned at a position inside the
triangular pyramid the Lab values of whose lattice points become
p4, p5, p6, and p7.
Here, regarding the Lab values of the CMY.fwdarw.Lab3D-LUT, when
its center value of C=M=Y=16.sup.th Lab value is taken as the
calculation reference point pc, the calculation target point is
taken as pin, the Lab values of the three lattice points set on the
surface of this CMY.fwdarw.Lab3D-LUT are taken as p4, p5, p6, and
p7, and the weighting factors in the Lab coordinate system set for
the calculation reference point p4 are taken as Y.sub.0, Y.sub.1,
and Y.sub.2, the relationship between the calculation reference
point p4 and the calculation target point pin is given by the
following Equation (16).
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times. ##EQU00003##
The weighting factors Y.sub.0, Y.sub.1, and Y.sub.2 are obtained
from this equation of relationship. At this time, if Y.sub.0>0,
Y.sub.1>0, and Y.sub.2>0, and also,
(Y.sub.0+Y.sub.1+Y.sub.2)<1, it implies that the calculation
target point is within the triangular pyramid. Therefore, the
corresponding CMY value (qout) is obtained from the weighting
factors Y.sub.0, Y.sub.1, and Y.sub.2 and the Lab values q4, q5,
q6, and q7 of the four lattice points. For example, the CMY value
(qout) corresponding to the input Lab value is computed using the
internal insertion processing mode using the Equation (17) and its
expanded equation to be described in FIG. 22 (internal insertion
processing mode). FIG. 22 is a diagram showing an example of
inclusion judgment in the CMY coordinate system. According to the
example of inclusion judgment shown in FIG. 22, when the CMY value
of the calculation reference point is taken as q4 in the CMY
coordinate system, and the CMY values of the different lattice
points are taken as q4, q5, q6, and q7, the internal insertion
processing mode computes the CMY value (qout) corresponding to the
input Lab values.
Here, regarding the CMY values of the CMY.fwdarw.Lab3D-LUT, when
the CMY value of that calculation target point is taken as qout,
and the CMY values of the addresses of the lattice points set on
the surface of that CMY.fwdarw.Lab3D-LUT are taken to be q5, q6,
and q7, the CMY value of the address set at the calculation
reference point is taken as q4, and the weighting factors in the
Lab coordinate system set for the calculation reference point q4
are taken as Y.sub.0, Y.sub.1, and Y.sub.2, the value of qout is
obtained by the following Equation (17).
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times. ##EQU00004##
Expanding this equation, we get:
qout.sub.--C=(q5.sub.--C-q4.sub.--C).times.Y.sub.0+(q6.sub.--C-q4.sub.--C-
).times.Y.sub.1+(q7.sub.--C-q4.sub.--C).times.Y.sub.2+q4.sub.--C
qout.sub.--M=(q5.sub.--M-q4.sub.--M).times.Y.sub.0+(q6.sub.--M-q4.sub.--M-
).times.Y.sub.1+(q7.sub.--M-q4.sub.--M).times.Y.sub.2+q4.sub.--M
qout.sub.--Y=(q5.sub.--Y-q4.sub.--Y).times.Y.sub.0+(q6.sub.--Y-q4.sub.--Y-
).times.Y.sub.1+(q7.sub.--Y-q4.sub.--Y).times.Y.sub.2+q4.sub.--Y
Further, it is possible to evaluate the Equation (16) and Equation
(17) beforehand and store the results in the ROM of the image
processing section 76.
After that, the operation moves on to Step A9, and a judgment is
made as to whether or not all the processing of computing the CMY
values for the Lab values set (input) earlier has been completed.
If this processing has not been completed, the operation returns to
Step A5 and the above processing is repeated. When the processing
of computing the CMY values for all the Lab values has been
completed, the RGB.fwdarw.CMY3D-LUT is generated.
Next, the operation moves on to Step A10, and the
RGB.fwdarw.CMY3D-LUT is converted into RGB.fwdarw.CMYK3D-LUT using
a GCR (pre-processing of ROM writing). In this example, each of the
CMY values of the RGB.fwdarw.CMY3D-LUT is converted into the
corresponding CMYK value using a GCR algorithm. Since this
conversion computation equation is as has been described earlier,
its explanation is omitted here. With this, it is possible to
generate the RGB.fwdarw.CMYK3D-LUT.
When the above processing has been completed, the operation moves
on to Step A11, and the ROM data Dout constituting the
RGB.fwdarw.CMYK3D-LUT is written in the ROM 40. For example, since
the ROM data Dout constituting the RGB.fwdarw.CMYK3D-LUT has been
stored in the RAM inside the control section 75, the data is
transferred from the RAM to the ROM writer 77, and is then written
into the ROM, etc. With this, it is possible to prepare a ROM 40 in
which the RGB.fwdarw.CMYK3D-LUT has been stored.
According to the information generation apparatus 70 and the method
of generating color conversion table as a preferred embodiment in
this manner, when converting the three color RGB input system color
image signals R, G, and B into the four color CMYK output system
color image signals C, M, Y, and K, it is possible to establish
correspondence between the color gamuts of the input system color
image signals and the color gamut of the output system color image
signals for each of the gradations including the maximum gradations
of those fundamental colors.
Therefore, the positions of the maximum gradations of fundamental
colors of the output system are moved to the positions of the
maximum gradations of fundamental colors of the input system and
the calculation target point is corrected. After that, compared to
the case of carrying out linear interpolation of the positions
other than that of the maximum gradations of the fundamental
colors, it is possible to reduce the extent of mixing of other
colors to a fundamental color. Because of this, it becomes possible
to reproduce not only fundamental colors but also colors other than
fundamental colors without color mixing, and hence it becomes
possible to generate a color conversion table that makes it
possible to reproduce high quality color images. Because there is
no color mixing, it has become possible to reduce the wasteful
consumption of toner.
Example of Evaluation of Fundamental Color Reproduction:
Next, the present preferred embodiment is compared with the
conventional example 1 and the conventional example 2 and the
ability to reproduce fundamental colors is evaluated. FIG. 23 is a
flow chart showing an example of evaluation of the fundamental
color reproduction capability. FIG. 24 to FIG. 31(F) are diagrams
supplementing this.
To begin with, the patch data is generated in Step G1 in the flow
chart shown in FIG. 23. For the patch data, the RGB value of a
fundamental color gradation on the input side is inputted to the
conventional example 1, the conventional example 2, and to the
RGB.fwdarw.3D-LUT generated using the color conversion table
according to the present preferred embodiment, and the CMYK value
corresponding to that address is used as the patch data.
Here, the conventional example 1 is the case when an
RGB.fwdarw.3D-LUT generated based on the flow chart of FIG. 32 is
used. The conventional example 2 is the case in which the
RGB.fwdarw.3D-LUT used is one that has been generated by omitting
the Step G42 after executing the Step G41 during the Step G4 in the
flow chart of FIG. 4. The present preferred embodiment is the case
in which the RGB.fwdarw.3D-LUT used is generated by executing both
the Step G41 and Step G42 in the flow chart of FIG. 4.
Next, in Step G2, the color patch based on the patch data is
printed out. In the example of constructing the color patch of the
colors CMYRGB shown in FIG. 24, the vertical numbers indicate the
sample numbers (1 to 9). The colors CMYRGB are arranged
sequentially from left to right in the color patch.
In the example of the relationship between the sample numbers 1 to
9 of the color patches of the color C shown in FIG. 25(A) and the
fundamental colors, regarding the color patch of the colors CMYRGB
shown in FIG. 24, the gradations of the colors G and B are both 255
(White), and the gradation of the color R is from 0 to 255 and
changes in 32 stages of gradations. In the example of the
relationship between the sample numbers 1 to 9 of the color patches
of the color R shown in FIG. 25(B) and the fundamental colors, the
gradation of the color R is 255 (White). The gradations of the
colors G and B are from 0 to 255 and change in 32 stages of
gradations.
In the example of the relationship between the sample numbers 1 to
9 of the color patches of the color M shown in FIG. 26(A) and the
fundamental colors, the gradations of the colors R and B are both
255 (White), and the gradation of the color G is from 0 to 255 and
changes in 32 stages of gradations. In the example of the
relationship between the sample numbers 1 to 9 of the color patches
of the color G shown in FIG. 26(B) and the fundamental colors, the
gradation of the color G is 255 (White). The gradations of the
colors R and B are from 0 to 255 and change in 32 stages of
gradations.
In the example of the relationship between the sample numbers 1 to
9 of the color patches of the color Y shown in FIG. 27(A) and the
fundamental colors, the gradations of the colors R and G are both
255 (White), and the gradation of the color B is from 0 to 255 and
changes in 32 stages of gradations. In the example of the
relationship between the sample numbers 1 to 9 of the color patches
of the color B shown in FIG. 27(B) and the fundamental colors, the
gradation of the color B is 255 (White). The gradations of the
colors R and G are from 0 to 255 and change in 32 stages of
gradations.
The colors of this type of color patches are measured in Step G3.
Color measurement data is obtained for the colors CMYRGB from this
color measurement. Next, in Step G4 the input values of the color
patch when the fundamental color gradations have been converted and
the CMYK output values are plotted in the form of a graph. At this
time, as is shown in FIG. 28, the graph is plotted by replacing the
sample numbers 1 to 9 with input values of 0 (white) to 8 (maximum
value) which are taken along the horizontal axis, and the measured
values, for example, the density is plotted along the vertical
axis. In this example, the output values of C, M, Y, and K are
indicated in percentage values. In addition, the output of the CMYK
values during bland (maximum density) are measured beforehand, and
these measured values are stored in the memory.
After that, in Step G5, an evaluation reference point is set in the
graph, and verification is done as to whether or not the color
mixing component in a fundamental color is below a prescribed
value. In the graph described above, the density of the color
causing color mixing is checked to see if it is, for example, less
than or equal to 10% of the CMYK value of the density at the time
of forming a bland (maximum density) patch. In the example shown in
FIG. 28, it is clear that the density of the color causing color
mixing is almost 6% of the CMYK value of the density at the time of
bland (maximum density) patch formation.
In FIG. 28, the horizontal axis is the input value (white to
maximum red). The vertical axis is the output value of the C, M, Y,
and K colors (%). Of course, the use of the vertical axis need not
be limited to these output values, but can also be used to
represent the amounts of toner of the colors C, M, Y, and K,
density, chroma, or 8-bit output values expressed as a particular
percentage. For example, the 8-bit value of 255 gradations is taken
as 100% and the 8-bit value 0 is taken as 0%.
In FIG. 28, the single dot and dash line indicating the color M and
the double dot and dash line indicating the color Y are almost
parallel, and the color R is being reproduced. Although the color C
indicated by a wavy line is bent, its density has been suppressed
to below 10%. From this, it is possible to evaluate the extent of
color mixing with respect to the color R (fundamental color). In
the reproduction of fundamental color gradations converted using
this type of color conversion method, the colors other than the
fundamental color being reproduced (the colors MYK in the case of
gradations of the color C, the colors CK in the case of gradations
of the color R) can be suppressed to below a prescribed level. The
term prescribed level here is less than 10% of the maximum density
of the color (can also be the saturation a* or b*) that has been
output with 255 gradations in the case of an 8-bit output, and
preferably within 5%, and hence it was clear that the color
reproduction is excellent.
FIG. 29(A) to FIG. 29(F) and FIG. 30(A) to FIG. 30(F) are drawings
showing examples of input-output conversions of the six fundamental
colors in the comparison examples of the conventional example 1 and
the conventional example 2, respectively. All these graphs have
nonlinearity.
In the case of reproducing the color C in the conventional example
1 shown in FIG. 29(A), the color Y is mixed in the base of color K.
In the case of reproducing the color M in the conventional example
1 shown in FIG. 29(B), the color C is mixed. In the case of
reproducing the color Y in the conventional example 1 shown in FIG.
29(C), the color C is mixed. Further, although there is no color
mixing in the case of reproducing the color R in the conventional
example 1 shown in FIG. 29(D), reproducing the color G in the
conventional example 1 shown in FIG. 29(E), and reproducing the
color B in the conventional example 1 shown in FIG. 29(F), the
graphs are non-linear.
FIG. 30(A) to FIG. 30(F) are drawings showing examples of color
mixing for the colors CMYRGB in the case of the conventional
example 2 in which only fundamental color adjustment has been made.
The conventional example 2 moves only the maximum gradations Max in
"formation of the CMY.fwdarw.Lab3D-LUT by adjustment of fundamental
colors", and linear interpolation is made in other cases. All the
graphs are non-linear.
In the case of reproducing the color C in the conventional example
2 shown in FIG. 30(A), although color mixing of the color Y has
been suppressed, color mixing of the color M is present. In the
case of reproducing the color M in the conventional example 2 shown
in FIG. 30(B), color mixing is present of the colors Y and C. In
the case of reproducing the color Y in the conventional example 2
shown in FIG. 30(C), color mixing of the colors M and C has been
suppressed. In the case of reproducing the color R in the
conventional example 2 shown in FIG. 30(D), color mixing of the
color C is present. In the case of reproducing the color G in the
conventional example 2 shown in FIG. 30(E), color mixing of the
color M is present. In the case of reproducing the color B in the
conventional example 2 shown in FIG. 30(F), color mixing of the
color Y is present.
FIG. 31(A) to FIG. 31(F) are drawings showing the input-output
conversion examples for the six fundamental colors according to the
present preferred embodiment. All the graphs have linearity. In the
present preferred embodiment, not only the maximum gradations Max
are moved, but also the gradations of the fundamental colors are
moved.
In the case of reproducing the color C in the present preferred
embodiment shown in FIG. 31(A), the color mixing of the colors Y
and M has been suppressed to zero, and color mixing has been
improved greatly compared to the conventional example 2. In the
case of reproducing the color M in the present preferred embodiment
shown in FIG. 31(B), the color mixing of the colors Y and C has
been suppressed to zero, and color mixing has been improved greatly
compared to the conventional example 2.
In the case of reproducing the color Y in the present preferred
embodiment shown in FIG. 31(C), the color mixing of the colors C
and M has been suppressed to zero, and color mixing has been
improved greatly compared to the conventional example 2. In the
case of reproducing the color R in the present preferred embodiment
shown in FIG. 31(D), the color mixing of the color C has been
suppressed to zero, and color mixing has been improved greatly
compared to the conventional example 2. In the case of reproducing
the color G in the present preferred embodiment shown in FIG.
31(E), the color mixing of the color M has been suppressed to zero,
and color mixing has been improve greatly compared to the
conventional example 2. In the case of reproducing the color B in
the present preferred embodiment shown in FIG. 31(F), the color
mixing of the color Y has been suppressed to zero, and color mixing
has been improve greatly compared to the conventional example
2.
As described above, by moving the calculation target point for
every gradation of the fundamental colors, during the reproduction
of the color R, the color C does not get mixed and the color is
formed only the colors M and Y, and hence the reproduction (output)
can be made purely (high accuracy) of the red color. Beautiful
appearance has been obtained because there is no color mixing. In
addition, it has been possible to reduce the wasteful consumption
of toners.
According to the information generation apparatus of the present
preferred embodiment, it is possible to establish correspondence
between the color gamut of the input system color image signals and
the color gamut of the output system color image signals for all
the gradations of the fundamental colors including the maximum
gradations of those fundamental colors.
Therefore, it is possible to eliminate color mixing of the
fundamental color gradations compared to the case of correcting the
calculation target point by moving the positions of the maximum
gradations of the fundamental colors of the output system to the
positions of the maximum gradations of the fundamental colors of
the input system and then carrying out linear interpolation of the
positions other than the maximum gradations. Because of this, it is
possible to reproduce colors without color mixing not only in the
case of fundamental colors but also in the case of colors other
than the fundamental colors.
According to a recording medium of the present preferred
embodiment, it is possible to carry out color conversion of M input
system color signals to N output system color signals without color
mixing.
According to an image processing apparatus according to the present
preferred embodiment, since a recording medium according to the
preferred embodiment has been provided, it is possible to carry out
color conversion of M input system color signals to N output system
color signals without color mixing. Because of this, it is possible
to provide an image processing apparatus that can reproduce color
images of a high quality. In addition, since there is no color
mixing, it is possible to reduce the toner consumption.
An embodiment of the present invention is ideally suitable for
application to color printers or color copying machines provided
with a three dimensional color conversion table, or to color
all-in-one units.
While the preferred embodiments of the present invention have been
described using specific terms, such description is for
illustrative purpose only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the appended claims.
* * * * *